1) If an accounting firm acts unethically (or goes further and actually breaks the law), then that can have massive negative impacts throughout society. For example, Arthur Andersen was a major accounting firm prior to the exposition of the Enron scandal. Upon discovery of the accounting irregularities, not only did both Enron and Arthur Andersen close (thus, eliminating thousands of jobs), but it caused Congress to issue the Sarbanes-Oxley Act of 2002 (a terrible piece of legislation which makes doing business even more expensive and complex without any material benefits). All of which could have been avoided if Arthur Andersen had just behaved ethically.
2) Conversely, accountants can have a positive impact on society. For example, by developing and employing a standardized method of accounting (ex., GAAP), we are able to compare and analyze financial statements from different businesses from different industries with relative ease. This means that investment and business decisions can be made more accurately and efficiently.